1. Field of the Invention
This invention relates to a superconducting material and a method of producing the same and more particularly, to a superconducting material made of a mixture of a fullerene and alkali metal and a method of producing the same.
2. Description of the Prior Art
As a superconducting material conventionally and practically realized, metal/alloy system and compound system ones or the like are known. The superconducting material has been widely applied for electronic devices such as the Josephson element and the like and for coils to be used in a superconducting magnet. Out of which, additionally to the applications for the superconducting quantum interference device (SQUID) magnetometer and other precision instrumentations in which the high sensitivity and precision characteristics and low noise performance of the Josephson junction is advantageously utilized, it has been largely expected to be applied for computers in which the high response and low power consumption characteristics of the Josephson junction are noticeably emphasized.
Under such a circumstance, a series of fullerenes consisting of cluster molecules of carbon were recognized to exist by Kuroto and Smalley in 1985 and then, its isolative synthesization was reported by Kratschmer and others in 1990. In this case, a fullerene is defined as a carbon material having a closed polyhedral structure consisting of pentagons and hexagons only. For this, such a series of materials that consist of cluster molecules of carbon having plural carbon atoms polyhedrally disposed in a soccer-ball pattern (a series of C.sub.a -system materials including C.sub.60 and C.sub.70) are known.
Recently, such a phenomenon that a mixture of fullerene and an alkali metal shows superconductivity upon passing through heat treatment has been confirmed and as a result, the fullerene has been suddenly attracting strong attention as a noticeable material for superconductivity in the future. A case in point is that such a phenomenon that a mixture of fullerene and potassium (K) shows superconductivity at or below 18 K. by heat treatment was reported by Hebard and others (see "Nature", Vol. 350, No. 18, pp. 600 to 601, 1991), which leads to the sudden recognition of the importance of fullerenes. Besides, such a phenomenon that a mixture of fullerene and rubidium (Rb) shows superconductivity at or below 28 K. by heat treatment was reported (see "Physical Review Letters", Vol. 66, No. 21, pp. 2830 to 2832, 1991). In these reports, a superconducting material is obtained by heat-treating a mixture of fullerene and alkali metal in a furnace and chemically structured so that electrons are doped from alkali metal atoms into carbon atoms of fullerene.
In addition, through a previous series of investigations, it has been though from such a trend that mixing cesium (Cs) as an alkali metal makes it possible to obtain further preferable results, and Holczer and others reported on their investigations that they tried using cesium as an alkali metal based on such trend (see "Science", vol. 252, pp. 1154 to 1157, 1991). In this case, however, according to their report, superconductivity is not observed on the cesium-added fullerene and it is considered that the reason for this is that cesium will be difficult to be entered as a dopant into the lattice space of the fullerene (C.sub.60) used.
A series of reports shown above reveal that an alkali-metal-added fullerene is possibly employed as a superconducting material in the future. In this case, however, the above-proposed superconducting materials have a superconducting transition temperature Tc as low as 28 K. and yet, a superconducting volume ratio, which is defined as a volume ratio of the region that shows superconductivity and the total region, is as low as 7%, which means that these values attained therethrough are unsatisfactory.
In addition, with the production methods proposed in these reports, fullerene and alkali metal are not sufficiently mixed with each other, thus being disadvantageous in that a superconducting material with a large superconducting volume ratio cannot be obtained.
Thus, an object of this invention is to provide a superconducting material consisting of a mixture of a fullerene and an alkali metal and which has higher superconducting transition temperature and larger superconducting volume ratio than those of the above-described conventional ones.
Another object of this invention is to provide a method of producing a superconducting material which consists of a mixture of a fullerene and an alkali metal and can be provided with a larger superconducting volume ratio than those of the above-described conventional ones.